A fundamental problem associated with using a conventional electrocardiograph (ECG) to monitor a patient's cardiac activity during MR imaging is the corruption of the ECG signal due to adverse electromagnetic effects. This effect is particularly pronounced in MR microscopy of small animals (e.g., laboratory rodents), where strong, rapidly-switching, magnetic field gradients are needed to obtain high spatial and temporal resolution, and the animal's ECG signal is less than a millivolt in amplitude. The spurious signals often resemble the QRS spike and can lead to erroneous cardiac gating. Furthermore, the artifacts often do not disappear until tens of milliseconds after the gradients turn off.
Several methods have been proposed to improve the quality of the ECG, and alternative measures of cardiac activity have been suggested. See, Felblinger et al, Magn. Res. Med., 32, 523–529 (1994); Lindberg et al, Med. Bio. Eng. Comp., 30, 533–537 (1992); and Legendre et al, Magn. Res. Med., 3, 953–957 (1986), the entire contents of each being incorporated hereinto expressly by reference. However, none of these conventional methods has been shown to provide reliable monitoring and gating ability in small rodents during cardiac MR microscopy. It is therefore towards fulfilling such a need that the present invention is directed.
Broadly, the present invention is embodied in noninvasive, MR-compatible methods and systems whereby mechanical cardiac activity is detected optically by movements in the esophagus and/or other anatomic structures affected by cardiac activity, such as, for example, the chest wall or blood vessels. More specifically, according to a particularly preferred embodiment of the present invention, esophageal compressions are used as a proxy for rhythmic cardiac activities. These esophageal compressions may be detected to provide a signal indicative of periods of cardiac activity and inactivity. The signal may be further processed so as to generate a trigger signal that may be input to a MR scanner. In such a manner, MR microscopy may be accomplished in such a manner so as to record images at desired specific phases of the cardiac cycle, for example to record images in synchrony with periods of cardiac inactivity. Moreover, since mechanical cardiac activity is detected and employed (i.e., by detecting physical movements in the esophagus and/or other anatomic structures affected by cardiac activity), instead of electrical activity as is employed in conventional techniques, the present invention is immune to electromagnetic interference during MR microscopy. As a result, robust cardiac signals may be monitored and gated during 2-dimensional and 3-dimensional in vivo microscopy. The present invention is therefore especially well suited for MR microscopy of small animals, such as laboratory mice and rats.
These aspects, as well as others, will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.